1. Field of the Invention
This invention relates to a distance measurement apparatus using an electromagnetic wave modulated in accordance with a pseudo random noise code.
2. Description of the Related Art
A prior-art distance measurement apparatus of a spread spectrum type which is mounted on an automotive vehicle measures the distance between the present vehicle and a preceding target object such as a preceding vehicle by using an electromagnetic wave modulated in accordance with a pseudo random noise code. Specifically, a beam of an electromagnetic wave whose amplitude is modulated in accordance with a pseudo random noise code of a predetermined bit length is emitted in a forward direction with respect to the body of the present vehicle. A moment of the transmission of the pseudo random noise code with the electromagnetic wave is memorized. The prior-art apparatus receives an echo beam caused by reflection of the forward electromagnetic-wave beam at a preceding target object. The received echo beam is converted into a binary echo electric signal. Calculation is made about the value of the correlation between the binary echo electric signal and the pseudo random noise code used for the modulation of the transmitted electromagnetic wave. A moment at which the calculated correlation value peaks is detected as a moment of the reception of the pseudo random noise code contained in the echo beam. The prior-art apparatus calculates the distance between the present vehicle and the preceding target object from the time interval between the moment of the transmission of the pseudo random noise code and the moment of the reception thereof, and also from the velocity of the electromagnetic wave.
In the prior-art apparatus, the pseudo random noise code has a sequence of bits, the number of which is predetermined. The correlation-value calculation is iterated at a period corresponding to one bit of the pseudo random noise code. Accordingly, the time interval between the moment of the transmission of the pseudo random noise code and the moment of the reception thereof is determined at a resolution corresponding to a 1-bit time interval (a 1-chip time interval). A resolution of the measured distance between the present vehicle and the preceding target depends on the resolution of the determination of the time interval. In the case where a clock frequency, the reciprocal of which corresponds to a 1-bit time interval, is equal to 20 MHz, the resolution of the measured distance between the present vehicle and the preceding target is equal to 7.5 m.
Japanese published unexamined patent application 5-312950 discloses a distance measurement apparatus which includes a maximum length code generator. A laser diode generates laser light modulated in accordance with a maximum length code produced by the maximum length code generator. A beam of the laser light is emitted toward an object. The apparatus of Japanese patent application 5-312950 includes a phase shifter which shifts the maximum length code by different values within a time interval corresponding to one bit of the maximum length code. The phase shifter outputs a plurality of shifted maximum length codes. The apparatus of Japanese patent application 5-312950 includes a photodiode which receives an echo beam caused by the reflection of the forward laser beam at the object. The photodiode converts the received echo beam into a corresponding echo electric signal. The values of the correlations between the echo electric signal and the shifted maximum length codes are calculated. The distance to the object is computed on the basis of the calculated correlation values.
Japanese published unexamined patent application 10-2963 discloses a distance measurement apparatus which uses an electromagnetic wave modulated in accordance with a pseudo random noise code. The apparatus of Japanese patent application 10-2963 includes a correlator which measures a time interval shorter than a 1-chip time interval (a 1-bit time interval) with respect to the pseudo random noise code.